Two-stage heat regenerating cryogenic refrigerator

ABSTRACT

A two-stage heat regenerating cryogenic refrigerator may include: a vacuum vessel; a first and second cylinder in the vessel; the second cylinder coaxially connected to the first cylinder; a 1st regenerator in the first cylinder and accommodating heat regenerating material (HRM) 1; and a second regenerator in the 2nd cylinder accommodating HRM 2, HRM 2 including HRM particles, each HRM particle including a metal element and a heat regenerating substance including an oxide or oxysulfide and having a maximum specific heat at ≤20 K of ≥0.3 J/cm3·K; each HRM particle including a 1st and 2nd region, the 2nd region being closer to each HRM particle&#39;s outer edge than the 1st, and the 2nd region having a higher metal element concentration than the 1st, the 1st and 2nd region containing the heat regenerating substance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuing application based upon U.S. applicationSer. No. 16/556,387, filed Aug. 30, 2019, which published as US2020/0300556 A1 on Sep. 24, 2020, and claims the benefit of priorityfrom Japanese Patent Application No. 2019-050475, filed on Mar. 18,2019, the entire contents of each of which are incorporated herein byreference.

FIELD

Embodiments described herein relate generally to a heat regeneratingmaterial particle, a regenerator, a refrigerator, a superconductingmagnet, a nuclear magnetic resonance imaging device, a nuclear magneticresonance device, a cryopump, and a single-crystal pulling device of amagnetic-field application type.

BACKGROUND

In a cryogenic refrigerator used for a cooling superconducting device,and the like, a heat regenerating material particle containing a heatregenerating substance having high volumetric specific heat in alow-temperature region is used. Here, specific heat per unit volume isdefined as volumetric specific heat. As the heat regenerating substance,for example, a metal such as lead (Pb) or bismuth (Bi), a rare earthcompound such as HoCu₂ or Er₃Ni, an oxide such as Ag₂O or Cu₂O, or anoxysulfide such as Gd₂O₂S is used.

In a cryogenic refrigerator, the regenerator is filled with a pluralityof heat regenerating material particles. For example, the cold isgenerated by exchanging heat between the heat regenerating materialparticles and the helium gas passing through the regenerator. The heatregenerating material particles to fill the regenerator are required tohave excellent characteristics such as high volumetric specific heat,high mechanical strength, and high heat transfer coefficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are explanatory drawings of a heat regenerating materialparticle of a first embodiment;

FIG. 2 is a schematic sectional view showing a main-part configurationof a refrigerator of a second embodiment;

FIG. 3 is a perspective view showing a schematic configuration of asuperconducting magnet of a third embodiment;

FIG. 4 is a sectional view showing a schematic configuration of anuclear magnetic resonance imaging device of a fourth embodiment;

FIG. 5 is a sectional view showing a schematic configuration of anuclear magnetic resonance device of a fifth embodiment;

FIG. 6 is a sectional view showing a schematic configuration of acryopump of a sixth embodiment; and

FIG. 7 is a perspective view showing a schematic configuration of asingle-crystal pulling device of a magnetic-field application type of aseventh embodiment.

DETAILED DESCRIPTION

A heat regenerating material particle of an embodiment contains: a heatregenerating substance having a maximum value of specific heat at atemperature of 20 K or less is 0.3 J/cm³·K or more; and one metalelement selected from the group consisting of calcium (Ca), magnesium(Mg), beryllium (Be), strontium (Sr), aluminum (Al), iron (Fe), copper(Cu), nickel (Ni), and cobalt (Co), wherein the heat regeneratingmaterial particle includes a first region and a second region, thesecond region is closer to an outer edge of the heat regeneratingmaterial particle than the first region, and the second region has ahigher concentration of the metal element than the first region.

Hereinafter, embodiments will be described with reference to thedrawings. In the following description, the same or similar members aredenoted by the same reference numeral, and the description of the memberor the like once described may be omitted as appropriate.

In the present specification, a cryogenic temperature means, forexample, a temperature range in which the superconducting phenomenon canbe used in an industrially useful manner. Such a temperature range is,for example, a temperature range of 20 K or less.

First Embodiment

A heat regenerating material particle of a first embodiment includes aheat regenerating substance having a maximum value of specific heat at atemperature of 20 K or less is 0.3 J/cm³·K or more; and one metalelement selected from the group consisting of calcium (Ca), magnesium(Mg), beryllium (Be), strontium (Sr), aluminum (Al), iron (Fe), copper(Cu), nickel (Ni), and cobalt (Co). The heat regenerating materialparticle includes a first region and a second region, the second regionis closer to an outer edge of the heat regenerating material particlethan the first region, and the second region has a higher concentrationof the metal element than the first region.

FIGS. 1A and 1B are explanatory diagrams of the heat regeneratingmaterial particle of the first embodiment. FIG. 1A is a schematicsectional view of the heat regenerating material particle. FIG. 1B is aview showing a concentration distribution of an added metal in the heatregenerating material particle.

Heat regenerating material particles 10 of the first embodiment areused, for example, in a refrigerator that achieves a cryogenictemperature of 5 K or less.

The shape of the heat regenerating material particle 10 is, for example,spherical. FIGS. 1A and 1B show a case where the heat regeneratingmaterial particle 10 is a true sphere. The particle size (D in FIG. 1A)of the heat regenerating material particle is, for example, equal to ormore than 50 μm and equal to or less than 500 μm.

The particle size D of the heat regenerating material particle 10 is anequivalent circle diameter. The equivalent circle diameter is thediameter of a perfect circle corresponding to an area of a figureobserved in an image such as an optical microscope image or a scanningelectron microscope image (SEM image). The particle size D of the heatregenerating material particle 10 can be determined, for example, byimage analysis of an optical microscope image or an SEM image.

The heat regenerating material particle 10 contains a heat regeneratingsubstance whose maximum value of specific heat at a temperature of 20 Kor less is 0.3 J/cm³·K or more.

The heat regenerating substance contains, for example, an oxide. Theheat regenerating substance contains, for example, an oxide as the maincomponent. The oxide contained in the heat regenerating substancecontains, for example, at least one of silver (Ag) and copper (Cu). Theoxide contained in the heat regenerating substance is, for example,silver oxide or copper oxide. The oxide contained in the heatregenerating substance is, for example, Ag₂O or Cu₂O.

The heat regenerating substance contains, for example, an oxysulfide.The heat regenerating substance contains, for example, an oxysulfide asthe main component. The oxysulfide contained in the heat regeneratingsubstance contains, for example, gadolinium (Gd). The oxysulfidecontained in the heat regenerating substance is, for example, gadoliniumoxysulfide. The oxysulfide contained in the heat regenerating substanceis, for example, Gd₂O₂S.

The heat regenerating substance contains, for example, a rare earthcompound as the main component. The rare earth compounds contained inthe heat regenerating substance are, for example, HoCu₂ and Er₃Ni.

The composition analysis of the heat regenerating substance can beperformed, for example, by energy dispersive X-ray spectroscopy (EDX) orwavelength dispersive X-ray spectrometry (WDX). Moreover, theidentification of the heat regenerating substance can be performed bythe powder X-ray diffraction method, for example.

The heat regenerating material particle 10 contains an added metal. Theadded metal is one metal element selected from the group consisting ofcalcium (Ca), magnesium (Mg), beryllium (Be), strontium (Sr), aluminum(Al), iron (Fe), copper (Cu), nickel (Ni), and cobalt (Co). The addedmetal is a metal that can be a polyvalent metal ion.

The heat regenerating material particle 10 has a low-concentrationregion 10 a (first region) and a high-concentration region 10 b (secondregion). The added-metal concentration in the high-concentration region10 b is higher than the added-metal concentration in thelow-concentration region 10 a.

The high-concentration region 10 b is closer to the outer edge of theheat regenerating material particle 10 than the low-concentration region10 a. The high-concentration region 10 b surrounds the low-concentrationregion 10 a. The low-concentration region 10 a is, for example, a regionincluding the center of the heat regenerating material particle 10, andthe high-concentration region 10 b is the outer peripheral region of thelow-concentration region 10 a.

The low-concentration region 10 a and the high-concentration region 10 beach contain the heat regenerating substance. At least in thehigh-concentration region 10 b, the heat regenerating substance and theadded metal are mixed. It is also possible to form a structure where theadded metal is not contained in the low-concentration region 10 a.

The added-metal concentration in the high-concentration region 10 b is,for example, equal to or more than 0.1 atom % and equal to or less than2.0 atom %.

The added-metal concentration in the high-concentration region 10 b is,for example, equal to or more than 1.03 times and equal to or less than10 times the added-metal concentration in the low-concentration region10 a. The distance (d in FIG. 1B) from the outer edge of the heatregenerating material particle 10 in the high-concentration region 10 b,where the added-metal concentration in the high-concentration region 10b is 1.03 times or more the added-metal concentration in thelow-concentration region 10 a is, for example, 1/20 or more of theparticle size D of the heat regenerating material particle 10. Thedistance d may be called a width of the high-concentration region 10 b.The distance d may be called a distance between the low-concentrationregion 10 a and the outer edge of the heat regenerating materialparticle 10.

The distance (d in FIG. 1B) from the outer edge of the heat regeneratingmaterial particle 10 in the high-concentration region 10 b, where theadded-metal concentration is 1.03 times or more the low-concentrationregion 10 a is, for example, 10 μm or more.

The added-metal concentration monotonously decreases, for example, fromthe outer edge of the heat regenerating material particle 10 toward thecenter.

The detection of the added metal contained in the heat regeneratingmaterial particle 10 and the measurement of the added-metalconcentration can be performed, for example, by wavelength dispersiveX-ray spectrometry (WDX). For example, it is possible to perform: themeasurement of the added-metal concentrations at a plurality oflocations by WDX from the outer edge of the heat regenerating materialparticle 10 toward the center; the determination of whether or not thehigh-concentration region 10 b close to the outer edge of the heatregenerating material particle 10 exists; the determination of theadded-metal concentration in the high-concentration region 10 b; thecalculation of a ratio of the added-metal concentration in thehigh-concentration region 10 b to the added-metal concentration in thelow-concentration region 10 a; and the calculation of the distance (d inFIG. 1B) from the outer edge of the heat regenerating material particle10 in the high-concentration region 10 b where the added-metalconcentration in the high-concentration region 10 b is 1.03 times ormore the added-metal concentration in the low-concentration region 10 a.Further, for example, the added-metal concentration in the heatregenerating material particle 10 is mapped by WDX, so that it ispossible to identify whether or not the high-concentration region 10 bsurrounds the low-concentration region 10 a.

Next, an example of a method for producing the heat regeneratingmaterial particle 10 of the first embodiment will be described.

First, a powder of the heat regenerating substance is added to anaqueous solution of alginic acid and mixed to prepare a slurry. Formixing the powder of the heat regenerating substance and the aqueoussolution of alginic acid, for example, a ball mill is used.

The prepared slurry is dropped into a gelling solution to gel theslurry. For dropping the slurry into the gelling solution, for example,a dropper, a burette, a pipette, a syringe, a dispenser, an inkjet, orthe like is used. By gelling the slurry, a spherical particle containinga heat regenerating substance are formed in the gelling solution.

The gelling solution contains an ionized added metal. As the time ofgelation elapses, the added metal penetrates from the outer edge of theparticle toward the center. By controlling the gelation time, theconcentration distribution of the added metal in the particle iscontrolled. That is, by controlling the gelation time, a distribution isformed in which the added-metal concentration in the central region ofthe particle is low and the added-metal concentration in the outerperipheral region of the particle is high.

From the viewpoint of forming the above distribution, the gelation timeis preferably as short as possible, so long as the shape of the particledoes not collapse in the subsequent step. The gelation time ispreferably within one hour, and more preferably within 30 minutes.

After the formation of the particle by gelation, the particle is washedwith pure water. By washing the particle, the added metal adsorbed onthe surface of the particle is removed.

After washing the particle, the particle is dried. After the drying ofthe particle, the particle is sintered to increase the mechanicalstrength of the particle and the density of the heat regeneratingsubstance in the particle.

The heat regenerating substance is, for example, silver oxide, copperoxide or gadolinium oxysulfide.

The aqueous solution of alginic acid is, for example, an aqueoussolution of sodium alginate, an aqueous solution of ammonium alginate,or an aqueous solution of potassium alginate.

The gelling solution is, for example, an aqueous solution of calciumlactate, an aqueous solution of calcium chloride, an aqueous solution ofmanganese (II) chloride, an aqueous solution of magnesium sulfate, anaqueous solution of beryllium sulfate, an aqueous solution of strontiumnitrate, an aqueous solution of aluminum chloride, an aqueous solutionof aluminum nitrate, an aqueous solution of aluminum lactate, an aqueoussolution of iron chloride (II), an aqueous solution of iron (III)chloride, an aqueous solution of copper (II) chloride, an aqueoussolution of nickel (II) chloride, and an aqueous solution of cobalt (II)chloride.

A combination of the heat regenerating substance, the aqueous solutionof alginic acid, and the gelling solution is optional. However, whensilver oxide as the heat regenerating substance is combined with theaqueous solution of calcium chloride as the gelling solution, silverchloride is generated, and hence this combination is excluded.

The heat regenerating material particle 10 of the first embodiment canbe produced by the above production method.

Next, the function and effects of the heat regenerating materialparticle 10 of the first embodiment will be described.

In a cryogenic refrigerator used for cooling a superconducting deviceand for other purposes, a regenerator is filled with a plurality of heatregenerating material particles. For example, the cold is generated byexchanging heat between the heat regenerating material particles and thehelium gas passing through the regenerator. The heat regeneratingmaterial particles to fill the regenerator are required to haveexcellent characteristics such as high volumetric specific heat, highmechanical strength, and high thermal conductivity.

The heat regenerating material particle 10 of the first embodimentcontains a heat regenerating substance whose maximum value of specificheat at a temperature of 20 K or less is 0.3 J/cm³·K or more. Therefore,the heat regenerating material particle 10 has high volumetric specificheat at a cryogenic temperature.

Moreover, the heat regenerating material particle 10 of the firstembodiment includes a high-concentration region 10 b having a highadded-metal concentration in the outer peripheral region of theparticle. The added metal has a function of promoting sintering of theparticle at the time of sintering during production of the heatregenerating material particle 10. Therefore, the high-concentrationregion 10 b has a high degree of sintering and high mechanical strength.Hence the heat regenerating material particle 10 has high mechanicalstrength.

Further, due to the high degree of sintering of the high-concentrationregion 10 b, the thermal conductivity of the high-concentration region10 b is high. Therefore, the heat regenerating material particle 10 hashigh thermal conductivity.

When the added-metal concentration increases, the volume ratio of theadded metal increases while the degree of sintering of the particleincreases, thereby causing a problem where the volume ratio of the heatregenerating substance decreases. In addition, the added metal forms acompound having low specific heat by reaction with the heat regeneratingsubstance. Thus, when the added-metal concentration becomes high, theheat regenerating substance changes into a compound having low specificheat, thereby causing a problem where the volume ratio of the heatregenerating substance decreases.

The heat regenerating material particle 10 of the first embodimentincludes the low-concentration region 10 a having a low added-metalconcentration in the central region of the particle. Therefore, in thecentral region of the particle, the decrease in the volume ratio of theheat regenerating substance due to the added element is reduced. Hencethe heat regenerating material particle 10 has high volumetric specificheat.

In the heat regenerating material particle 10 of the first embodiment,the mechanical strength and the thermal conductivity are improved byproviding the high-concentration region 10 b having a high added-metalconcentration in the outer peripheral region of the particle. On theother hand, the volumetric specific heat is improved by providing thelow-concentration region 10 a in the central region of the particle. Theheat regenerating material particle 10 of the first embodiment has highvolumetric specific heat, high mechanical strength, and high thermalconductivity by optimizing the concentration distribution of the addedelement in the particle.

The optimization of the concentration distribution of the added elementin the heat regenerating material particles 10 can be achieved byoptimizing the gelation time during production of the heat regeneratingmaterial particle 10. More specifically, the optimization can beachieved by shortening the gelation time as much as possible. When thegelation time is excessively long, the concentration of the addedelement in the heat regenerating material particle 10 becomes uniform tocause deterioration in characteristics. The gelation time is preferablywithin one hour, and more preferably within 30 minutes.

From the viewpoint of increasing the mechanical strength and the thermalconductivity of the heat regenerating material particle 10, theadded-metal concentration in the high-concentration region 10 b ispreferably 0.1 atom % or more, and more preferably 0.2 atom % or more.

From the viewpoint of achieving high mechanical strength, high thermalconductivity, and high volumetric specific heat of the heat regeneratingmaterial particle 10, the added-metal concentration inhigh-concentration region 10 b is 1.03 times or more the added-metalconcentration in low-concentration region 10 a, more preferably 1.05times or more, further preferably 1.1 times or more, and most preferably1.2 times or more.

From the viewpoint of increasing the mechanical strength and the thermalconductivity of the heat regenerating material particle 10, the distance(d in FIG. 1B) from the outer edge of the heat regenerating materialparticle 10 in the high-concentration region 10 b, where the added-metalconcentration in the high-concentration region 10 b is 1.03 times ormore the added-metal concentration in the low-concentration region 10 ais preferably 1/20 or more of the particle size D (D in FIG. 1A) of theheat regenerating material particle 10, and more preferably 1/10 or moreof the particle size D.

From the viewpoint of increasing the mechanical strength and the thermalconductivity of the heat regenerating material particle 10, the distancefrom the outer edge of the heat regenerating material particle 10 in thehigh-concentration region 10 b where the added-metal concentration is1.1 times or more the added-metal concentration (d in FIG. 1B) ispreferably 10 μm or more, and more preferably 20 μm or more.

From the viewpoint of achieving high mechanical strength, high thermalconductivity, and high volumetric specific heat of the heat regeneratingmaterial particle 10, the added-metal concentration preferably decreasesmonotonously from the outer edge of the heat regenerating materialparticle 10 toward the center.

From the viewpoint of increasing the mechanical strength and the thermalconductivity of the heat regenerating material particle 10, the addedelement is preferably calcium (Ca) having a high ability to promotesintering.

Hereinafter, examples and comparative examples of the heat regeneratingmaterial particle 10 of the first embodiment and evaluation results ofthose will be described.

According to the method for producing a heat regenerating materialparticle of the first embodiment described above, a heat regeneratingmaterial particle was produced. Silver oxide, copper oxide, andgadolinium oxysulfide were used as a heat regenerating substance, anaqueous solution of sodium alginate was used as an aqueous solution ofalginic acid, an aqueous solution of calcium lactate was used as agelling solution, and a syringe was used as a dropping method. Theconcentration of the aqueous solution of sodium alginate, the amount ofthe heat regenerating substance to be added to the aqueous solution ofsodium alginate, the concentration of the aqueous solution of calciumlactate, and the gelation time were appropriately changed to obtain heatregenerating material particles shown in Table 1 (Examples 1 to 18,Comparative Example 1 to 3).

In order to evaluate the mechanical strength required of heatregenerating material particles when the regenerator constituting therefrigerator is filled with the heat regenerating material particles andthe refrigerator operates, each cylindrical container having a diameterof φ15 mm and a height of 5 cm is filled with the produced heatregenerating material particles, and a single vibration with a maximumacceleration of 200 m/s² was applied to the container 1,000 times. Thepresence or absence of a broken heat regenerating material particle as aresult of the above is shown. It can be seen that when the concentration(atom %) of the metal element in the second region falls below 0.1, thesintering does not proceed sufficiently and the mechanical strength ofthe heat regenerating material particle decreases.

In addition, the volumetric specific heat at 20 K is measured andcompared with the volumetric specific heat inherent to the heatregenerating substance, that is, the volumetric specific heat at thetime when the heat regenerating substance volume ratio is 100 vol %, toevaluate the volumetric specific heat reduction rate of the heatregenerating material particles obtained this time (Examples 1 to 18 andComparative Examples 1 to 3) with respect to the volumetric specificheat at the time when the heat regenerating substance volume ratio is100 vol %. From this result, when the concentration ratio (C2/C1) of theconcentration (C2) of the metal element in the second region to theconcentration (C1) of the metal element in the first region falls below1.03, the metal element extends throughout the heat regeneratingmaterial particle and the ratio of the metal element becomes excessive,so that a decrease in volumetric specific heat is observed. Then, it isfound that, when the concentration ratio becomes 1, that is, when theconcentration (C1) of the metal element in the first region and theconcentration (C2) of the metal element in the second region becomeequal, a decrease in volumetric specific heat exceeds 5% which is notpreferred in practical use.

TABLE 1 Conc. Conc. (C1) of (C2) of metal metal element element SpecificCold in in Conc. heat accumulating first second ration Destructivereduction substance region region (C2/C1) particle rate @20K Example 1Silver oxide 0.126% 0.130% 1.03 No less than 1% Example 2 Silver oxide0.147% 0.154% 1.05 No less than 1% Example 3 Silver oxide 0.193% 0.212%1.1 No less than 1% Example 4 Silver oxide 0.100% 0.102% 1.02 No 2%Example 5 Silver oxide 0.229% 0.231% 1.01 No 4% Example 6 Silver oxide0.060% 0.063% 1.05 Yes less than 1% Comparative Silver oxide 0.125%0.125% 1 No 6% Example 1 Example 7 Copper oxide 0.127% 0.131% 1.03 Noless than 1% Example 8 Copper oxide 0.107% 0.112% 1.05 No less than 1%Example 9 Copper oxide 0.066% 0.069% 1.05 Yes less than 1% Example 10Copper oxide 0.106% 0.108% 1.02 No 2% Example 11 Copper oxide 0.143%0.144% 1.01 No 4% Example 12 Copper oxide 0.145% 0.159% 1.1 No less than1% Comparative Copper oxide 0.138% 0.138% 1 No 6% Example 2 Example 13Gadolinium 0.191% 0.197% 1.03 No less than 1% oxysulfide Example 14Gadolinium 0.230% 0.242% 1.05 No less than 1% oxysulfide Example 15Gadolinium 0.179% 0.197% 1.1 No less than 1% oxysulfide Example 16Gadolinium 0.184% 0.188% 1.02 No 2% oxysulfide Example 17 Gadolinium0.128% 0.129% 1.01 No 4% oxysulfide Example 18 Gadolinium 0.052% 0.055%1.05 Yes less than 1% oxysulfide Comparative Gadolinium 0.135% 0.135% 1No 6% Example 3 oxysulfide

As described above, of the first embodiment, it is possible to achieve aheat regenerating material particle having excellent characteristicssuch as high volumetric specific heat, high mechanical strength, and ahigh heat transfer coefficient.

Second Embodiment

A refrigerator of a second embodiment is a refrigerator including aregenerator filled with a plurality of heat regenerating materialparticles of the first embodiment. Hereinafter, a part of thedescription of contents overlapping with those of the first embodimentwill be omitted.

FIG. 2 is a schematic sectional view showing a main-part configurationof the refrigerator of the second embodiment. The refrigerator of thesecond embodiment is a two-stage heat regenerating cryogenicrefrigerator 100 used for cooling a superconducting device or the like.

The heat regenerating cryogenic refrigerator 100 includes a firstcylinder 111, a second cylinder 112, a vacuum vessel 113, a firstregenerator 114, a second regenerator 115, a first seal ring 116, asecond seal ring 117, a first heat regenerating material 118, a secondheat regenerating material 119, a first expansion chamber 120, a secondexpansion chamber 121, a first cooling stage 122, a second cooling stage123, and a compressor 124.

The heat regenerating cryogenic refrigerator 100 includes a vacuumvessel 113 in which the first cylinder 111 having a large diameter andthe second cylinder 112 having a small diameter and coaxially connectedto the first cylinder 111 are installed. The first regenerator 114 isdisposed in the first cylinder 111 so as to be able to reciprocate. Inthe second cylinder 112, the second regenerator 115 which is an exampleof the regenerator of the second embodiment is disposed so as to be ableto reciprocate.

A first seal ring 116 is disposed between the first cylinder ill and thefirst regenerator 114. A second seal ring 117 is disposed between thesecond cylinder 112 and the second regenerator 115.

The first regenerator 114 accommodates a first heat regeneratingmaterial 118 such as a Cu mesh. The second regenerator 115 is filledwith a plurality of heat regenerating material particles 10 of the firstembodiment as the second heat regenerating material 119.

The first regenerator 114 and the second regenerator 115 each have apassage of a working medium provided in a gap between the first heatregenerating material 118 and the second heat regenerating material 119,or the like. The working medium is helium gas.

The first expansion chamber 120 is provided between the firstregenerator 114 and the second regenerator 115. Further, the secondexpansion chamber 121 is provided between the second regenerator 115 andthe end wall of the second cylinder 112. The first cooling stage 122 isprovided at the bottom of the first expansion chamber 120.

In addition, the second cooling stage 123 having a temperature lowerthan that of the first cooling stage 122 is formed at the bottom of thesecond expansion chamber 121.

The first expansion chamber 120 is provided between the firstregenerator 114 and the second regenerator 115. Further, the secondexpansion chamber 121 is provided between the second regenerator 115 andthe end wall of the second cylinder 112. The first cooling stage 122 isprovided at the bottom of the first expansion chamber 120. In addition,the second cooling stage 123 having a temperature lower than that of thefirst cooling stage 122 is formed at the bottom of the second expansionchamber 121.

A high-pressure working medium is supplied from the compressor 124 tothe above-described two-stage heat regenerating cryogenic refrigerator100. The supplied working medium passes through the first heatregenerating material 118 accommodated in the first regenerator 114 andreaches the first expansion chamber 120. The supplied working mediumthen passes through the second heat regenerating material 119accommodated in the second regenerator 115 and reaches the secondexpansion chamber 121.

At this time, the working medium is cooled by supplying thermal energyto the first heat regenerating material 118 and the second heatregenerating material 119. The working medium having passed through thefirst heat regenerating material 118 and the second heat regeneratingmaterial 119 is expanded in the first expansion chamber 120 and thesecond expansion chamber 121 to generate the cold. Then, the firstcooling stage 122 and the second cooling stage 123 are cooled.

The expanded working medium flows in the opposite direction through thefirst heat regenerating material 118 and the second heat regeneratingmaterial 119. The working medium is discharged after receiving thermalenergy from the first heat regenerating material 118 and the second heatregenerating material 119. The heat regenerating cryogenic refrigerator100 is configured such that in such a process as above, as the heatrecovery effect becomes better, the thermal efficiency of the workingmedium cycle is improved, and a lower temperature is achieved.

As described above, according to the second embodiment, a refrigeratorwith excellent characteristics can be achieved by using the heatregenerating material particles with excellent characteristics.

Third Embodiment

A superconducting magnet of a third embodiment includes the refrigeratorof the second embodiment. Hereinafter, a part of the description ofcontents overlapping with those of the second embodiment will beomitted.

FIG. 3 is a perspective view showing a schematic configuration of thesuperconducting magnet of the third embodiment. The superconductingmagnet of the third embodiment is a superconducting magnet 300 for amagnetically levitated train which includes the heat regeneratingcryogenic refrigerator 100 of the second embodiment.

The superconducting magnet 300 for a magnetically levitated trainincludes a superconducting coil 301, a liquid helium tank 302 forcooling the superconducting coil 301, a liquid nitrogen tank 303 forpreventing volatilization of the liquid helium tank 302, a laminatedheat insulating material 305, a power lead 306, a permanent currentswitch 307, and the heat regenerating cryogenic refrigerator 100.

According to the third embodiment, a superconducting magnet withexcellent characteristics can be achieved by using the refrigerator withexcellent characteristics.

Fourth Embodiment

A nuclear magnetic resonance imaging device of a fourth embodimentincludes the refrigerator of the second embodiment. Hereinafter, a partof the description of contents overlapping with those of the secondembodiment will be omitted.

FIG. 4 is a sectional view showing a schematic configuration of thenuclear magnetic resonance imaging device of the fourth embodiment. Thenuclear magnetic resonance imaging (MRI) device of the fourth embodimentis a nuclear magnetic resonance imaging device 400 including the heatregenerating cryogenic refrigerator 100 of the second embodiment.

The nuclear magnetic resonance imaging device 400 includes asuperconducting static magnetic field coil 401 for applying a spatiallyuniform and temporally stable static magnetic field to a human body, acorrection coil (not shown) for correcting nonuniformity of a generatedmagnetic field, a gradient magnetic field coil 402 for giving a magneticfield gradient to a measurement region, a radio wavetransmitting/receiving probe 403, a cryostat 405, and a radiationadiabatic shield 406. The heat regenerating cryogenic refrigerator 100is used for cooling the superconducting static magnetic field coil 401.

According to the fourth embodiment, a nuclear magnetic resonance imagingdevice with excellent characteristics can be achieved by using therefrigerator with excellent characteristics.

Fifth Embodiment

A nuclear magnetic resonance device of a fifth embodiment includes therefrigerator of the second embodiment. Hereinafter, a part of thedescription of contents overlapping with those of the second embodimentwill be omitted.

FIG. 5 is a sectional view showing a schematic configuration of thenuclear magnetic resonance device of the fifth embodiment. The nuclearmagnetic resonance (NMR) device of the fifth embodiment is a nuclearmagnetic resonance device 500 including the heat regenerating cryogenicrefrigerator 100 of the second embodiment.

The nuclear magnetic resonance device 500 includes a superconductingstatic magnetic field coil 502 for applying a magnetic field to a samplesuch as an organic substance contained in a sample tube 501, ahigh-frequency oscillator 503 for applying a radio wave to the sampletube 501 in the magnetic field, and an amplifier 504 for amplifying aninduced current generated in a coil (not shown) on the periphery of thesample tube 501. In addition, the heat regenerating cryogenicrefrigerator 100 for cooling the superconducting static magnetic fieldcoil 502 is provided.

According to the fifth embodiment, a nuclear magnetic resonance devicewith excellent characteristics can be achieved by using the refrigeratorwith excellent characteristics.

Sixth Embodiment

A cryopump of a sixth embodiment includes the refrigerator of the secondembodiment. Hereinafter, a part of the description of contentsoverlapping with those of the second embodiment will be omitted.

FIG. 6 is a sectional view showing a schematic configuration of thecryopump of the sixth embodiment. The cryopump of the sixth embodimentis a cryopump 600 including the heat regenerating cryogenic refrigerator100 of the second embodiment.

The cryopump 600 includes a cryopanel 601 for condensing or adsorbinggas molecules, the heat regenerating cryogenic refrigerator 100 forcooling the cryopanel 601 to a predetermined cryogenic temperature, ashield 603 provided between the cryopanel 601 and the heat regeneratingcryogenic refrigerator 100, a baffle 604 provided at an inlet port, anda ring 605 for changing an exhaust rate of argon, nitrogen, hydrogen, orthe like.

According to the sixth embodiment, a cryopump with excellentcharacteristics can be achieved by using the refrigerator with excellentcharacteristics.

Seventh Embodiment

A single-crystal pulling device of a magnetic-field application type ofa seventh embodiment includes the refrigerator of the second embodiment.Hereinafter, a part of the description of contents overlapping withthose of the second embodiment will be omitted.

FIG. 7 is a perspective view showing a schematic configuration of thesingle-crystal pulling device of the magnetic-field application type ofthe seventh embodiment. The single-crystal pulling device of themagnetic-field application type of the seventh embodiment is asingle-crystal pulling device 700 of a magnetic-field application typeincluding the heat regenerating cryogenic refrigerator 100 of the secondembodiment.

The single-crystal pulling device 700 of the magnetic-field applicationtype includes a single-crystal pulling unit 701 having a raw-materialmelting crucible, a heater, a single-crystal pulling mechanism, and thelike, a superconducting coil 702 for applying a static magnetic field toa raw material melt, a lifting mechanism 703 of the single-crystalpulling unit 701, a current lead 705, a heat shield plate 706, and ahelium container 707. The heat regenerating cryogenic refrigerator 100is used for cooling the superconducting coil 702.

According to the seventh embodiment, a single-crystal pulling device ofa magnetic-field application type with excellent characteristics can beachieved by using the refrigerator with excellent characteristics.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the heat regenerating materialparticle, the regenerator, the refrigerator, the superconducting magnet,the nuclear magnetic resonance imaging device, the nuclear magneticresonance device, the cryopump, and the single-crystal pulling device ofthe magnetic-field application type described herein may be embodied ina variety of other forms; furthermore, various omissions, substitutionsand changes in the form of the devices and methods described herein maybe made without departing from the spirit of the inventions. Theaccompanying claims and their equivalents are intended to cover suchforms or modifications as would fall within the scope and spirit of theinventions.

What is claimed is:
 1. A two-stage heat regenerating cryogenicrefrigerator comprising: a vacuum vessel; a first cylinder disposed inthe vessel; a second cylinder disposed in the vessel, the secondcylinder coaxially connected to the first cylinder; a first regeneratordisposed in the first cylinder, the first regenerator accommodating afirst heat regenerating material; and a second regenerator disposed inthe second cylinder, the second regenerator accommodating a second heatregenerating material, the second heat regenerating material including aplurality of heat regenerating material particles, each of the heatregenerating material particles including a heat regenerating substancehaving a maximum value of specific heat at a temperature of 20 K or lessof 0.3 J/cm3·K or more and one metal element selected from the groupconsisting of calcium (Ca), magnesium (Mg), beryllium (Be), strontium(Sr), aluminum (Al), iron (Fe), copper (Cu), nickel (Ni), and cobalt(Co); wherein each of the heat regenerating material particles includesa first region and a second region, the second region is closer to anouter edge of each of the heat regenerating material particles than thefirst region, and the second region has a higher concentration of themetal element than the first region, the first region and the secondregion contain the heat regenerating substance, and the heatregenerating substance contains a component in a form of an oxide or anoxysulfide.
 2. The two-stage heat regenerating cryogenic refrigeratoraccording to claim 1, wherein the second cylinder has a diameter smallerthan a diameter of the first cylinder.
 3. The two-stage heatregenerating cryogenic refrigerator according to claim 1, wherein thefirst heat regenerating material is a Cu mesh.
 4. The two-stage heatregenerating cryogenic refrigerator according to claim 1, wherein thesecond region surrounds the first region.
 5. The two-stage heatregenerating cryogenic refrigerator according to claim 1, wherein aconcentration of the metal element in the second region is 0.1 atom % ormore.
 6. The two-stage heat regenerating cryogenic refrigeratoraccording to claim 1, wherein a concentration of the metal element inthe second region is 1.03 times or more a concentration of the metalelement in the first region.
 7. The two-stage heat regeneratingcryogenic refrigerator according to claim 6, wherein a distance from theouter edge of the each of the heat regenerating material particles inthe second region is 1/20 or more of a particle size of the each of theheat regenerating material particles.
 8. The two-stage heat regeneratingcryogenic refrigerator according to claim 6, wherein a distance from theouter edge of the each of the heat regenerating material particles inthe second region is 10 μm or more.
 9. The two-stage heat regeneratingcryogenic refrigerator according to claim 1, wherein a concentration ofthe metal element monotonously decreases from the outer edge of the eachof the heat regenerating material particles toward a center of the eachof the heat regenerating material particles.
 10. The two-stage heatregenerating cryogenic refrigerator according to claim 1, wherein aparticle size of the each of the heat regenerating material particles isequal to or more than 50 μm and equal to or less than 500 μm.
 11. Thetwo-stage heat regenerating cryogenic refrigerator according to claim 1,wherein the heat regenerating substance contains a component in a formof silver oxide, copper oxide, or gadolinium oxysulfide.
 12. A two-stageheat regenerating cryogenic refrigerator comprising: a vacuum vessel; afirst cylinder disposed in the vessel; a second cylinder disposed in thevessel, the second cylinder coaxially connected to the first cylinder; afirst regenerator disposed in the first cylinder, the first regeneratoraccommodating a first heat regenerating material; and a secondregenerator disposed in the second cylinder, the second regeneratoraccommodating a second heat regenerating material, the second heatregenerating material including a plurality of heat regeneratingmaterial particles, each of the heat regenerating material particlesincluding a heat regenerating substance having a maximum value ofspecific heat at a temperature of 20 K or less of 0.3 J/cm3·K or moreand one metal element selected from the group consisting of calcium(Ca), magnesium (Mg), beryllium (Be), strontium (Sr), aluminum (Al),iron (Fe), nickel (Ni), and cobalt (Co); wherein the each of the heatregenerating material particles includes a first region and a secondregion, the second region is closer to an outer edge of the each of theheat regenerating material particles than the first region, and thesecond region has a higher concentration of the metal element than thefirst region.
 13. The two-stage heat regenerating cryogenic refrigeratoraccording to claim 12, wherein the second cylinder has a diametersmaller than a diameter of the first cylinder.
 14. The two-stage heatregenerating cryogenic refrigerator according to claim 12, wherein thefirst heat regenerating material is a Cu mesh.
 15. The two-stage heatregenerating cryogenic refrigerator according to claim 12, wherein theheat regenerating substance contains a component in a form of an oxideor an oxysulfide.
 16. The two-stage heat regenerating cryogenicrefrigerator according to claim 12, wherein the heat regeneratingsubstance contains a component in a form of silver oxide, copper oxide,or gadolinium oxysulfide.
 17. The two-stage heat regenerating cryogenicrefrigerator according to claim 12, wherein the second region surroundsthe first region.
 18. The two-stage heat regenerating cryogenicrefrigerator according to claim 12, wherein a concentration of the metalelement in the second region is 0.1 atom % or more.
 19. The two-stageheat regenerating cryogenic refrigerator according to claim 12, whereina concentration of the metal element in the second region is 1.03 timesor more a concentration of the metal element in the first region. 20.The two-stage heat regenerating cryogenic refrigerator according toclaim 19, wherein a distance from the outer edge of the each of the heatregenerating material particles in the second region is 1/20 or more ofa particle size of the each of the heat regenerating material particles.